Probably Thunderbird, but maybe consider looking at Postbox. Their latest version includes a variety of colourful themes, and it seems that customising the visu

Probably Thunderbird, but maybe consider looking at Postbox. Their latest version includes a variety of colourful themes, and it seems that customising the visual appearance is very important to IncrediMail users. (For context, I have a small business making Photoshop plug-ins, and a lot of my customer support emails were from IncrediMail users. I've seen a few bright yellow Comic Sans emails on purple background with animated smiley-face GIFs. The point of IncrediMail seems to have been to make emails more like the penpal letter writing stationery kits you can find at a scrapbooking store. The customers used my Photoshop plug-in to design plaid backgrounds that they used as stationery in their outgoing IncrediMail HTML emails. I did find that the IncrediMail users were among the friendliest and nicest when dealing with their support emails.)